1. Field of the Invention
The present invention relates to the treatment of aqueous waste streams containing organoboron compounds in order to convert the organoboron compounds to boric acid and organic compounds. In particular, this invention relates to the hydrolysis of phenylated boron compounds to boric acid and benzene.
2. Description of Related Art
Organoboron compounds, such as triphenylborane, are suitable for use as promoters in the hydrocyanation of butadiene. Arylboron compounds used in hydrocyanation reactions can be prepared by the process described in U.S. Pat. No. 4,046,815 and 4,076,756. The preparation of such compounds can produce an aqueous waste stream that contains various amounts of organoboron compounds. For environmental reasons, it is desirable to reduce the level of organoboron compounds in such streams.
U.S. Pat. No. 5,709,841 discloses a process in which aqueous waste streams containing organoboron compounds of the formula R3B, R4Bxe2x88x921, and R2BOH are treated to hydrolyze the compounds to boric acid and the corresponding hydrocarbons. The process involves treatment of the waste streams to adjust the pH of the stream to a value between 5 and 9 and elevating the temperature to at least 150xc2x0 C., at a pressure sufficiently high to prevent substantial evaporation of the aqueous solution. This process may not be sufficient for complete R4Bxe2x88x921 hydrolysis within a reasonable period of time.
U.S. Pat. No. 4,840,765 discloses a process for acid hydrolysis of 4PBS (sodium tetraphenylborate) contained in nuclear waste. The hydrolysis is performed in the presence of a copper(II) cation catalyst and formic acid.
The present invention is a process for treating an aqueous waste steam containing an organoboron compound of the formula R4Bxe2x88x921M+, comprising adjusting the pH of said stream to a value between 5 and 11, and contacting said stream with hydrogen peroxide at a temperature of at least 150xc2x0 C.; wherein: R is selected from the group consisting of phenyl, ortho-tolyl, para-tolyl, naphthyl, para-methoxyphenyl, para-aminophenyl and biphenyl, and wherein R is optionally substituted with Br or Cl, and M+ is selected from the group consisting of Na+, K+, NH4+, Li+, H+, and Cs+.
Aqueous waste streams containing R4Bxe2x88x921M+ may result from preparing triphenylboron by the method described in U.S. Pat. No. 4,046,815. Organoboron compounds of the formula R3B, R2BOH, and RB(OH)2 also are typical components of such waste streams. The R4Bxe2x88x921 compound requires a counter ion, M+. Typically M+ is Na+. The organic groups (R) of R4Bxe2x88x921M+ can be the same or different. R can be aryl or substituted aryl having 6 to 12 carbon atoms. Examples of such R groups are phenyl, orthotolyl, paratolyl, naphthyl, para-methoxyphenyl, para-aminophenyl, biphenyl, chlorophenyl, and bromophenyl. Such waste streams will usually contain, in addition to the organoboron compound(s), (1) alkali metal halides, (2) lower alkyl alcohols (ie, those with 1 to 6 carbon atoms), and (3) boric acid.
The aforementioned waste streams containing the organoboron compounds can be treated to hydrolyze the organoboron compounds to boric acid and an organic compound, and then subjected to a distillation to remove the resulting organic compound (benzene, for example, in the case of phenylated boron compounds). The resulting product is then amenable to biotreatment. It is important that substantially all of the organoboron content is hydrolyzed prior to biotreatment. Compounds of the formula R4Bxe2x88x921M+ (sodium tetraphenylborate, for example) are particularly difficult to hydrolyze. Sodium tetraphenylborate can inhibit wastewater biotreatment systems.
The addition of hydrogen peroxide to the hydrolysis reaction increases the extent of hydrolysis of the R4Bxe2x88x921 compounds. This makes the hydrolysis product more amenable to bio-processing.
Hydrogen peroxide is preferred over other oxygen sources because of its high solubility which allows 200 ppm or greater levels to be used in the process. Preferably, the concentration of hydrogen peroxide is at least 200 ppm. More preferably, the concentration is greater than 2000 ppm.
The hydrolysis reaction can be performed at a pH of 5 to 11. The temperature should be at least 150xc2x0 C. Preferably, the temperature is 200 to 250xc2x0 C. The preferred pressure of the hydrolysis is 150 to 4000 psig (1.14 to 27.7 MPa). Preferably, the pressure is chosen to minmize vaporization of the aqueous solution.
Optionally, a metal catalyst may be used to promote the hydrolysis of R4Bxe2x88x921 compounds. However, due to potential bio-toxicity issues, it is preferred that no such catalyst be used.
The present process is amenable to the aforementioned R4Bxe2x88x921M+ compounds in which M+ is K+, NH4+, Li+, H+, and Cs+. It is also applicable to organoboron compounds in which the R groups are substituted with Br or Cl.